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T-Mobile, drops from LTE to 3G when making a call

4th March 2014, 11:52 PM |#1  
OP Senior Member
Flag Atlanta
Thanks Meter: 20
Just wondering if this happens to anyone else. My N5 was originally on Sprint but just recently switched to TMO. I have noticed that when on the TMO network my phone will drop to 3G whenever I make or receive a call. As soon as the call is over it switches back to LTE.

If I put my Sprint SIM back in and use their network it seems to hold LTE while on the call.

This is not scientific and am basing this observation from the network signal indicator on the top status bar. And as far as I can tell it isn't affecting performance. More of a curiosity at this point.

Sent from my Nexus 5 using xda app-developers app
4th March 2014, 11:58 PM |#2  
simms22's Avatar
Recognized Contributor
Thanks Meter: 24,028
this is how its supposed to work. phone calls cant go through the lte signal(in this case, with tmibile) to the phone, so it drops to 3G(H).
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5th March 2014, 12:37 AM |#3  
Senior Member
NYC & Germany
Thanks Meter: 455
LTE in it's current implementation is a DATA ONLY service so when you make voice calls it drops that connection and connects the call. Since 3G is Voice/Data it shows the 3G/H connection available to you at that time. When you hang up it goes back to LTE.

With Sprint it uses CDMA so theres no simultaneous option with that so LTE is kept alive in order to deliver data at the sametime.

All expected behavior as already mentioned
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5th March 2014, 01:24 AM |#4  
OP Senior Member
Flag Atlanta
Thanks Meter: 20
Thanks for the info. Makes perfect sense how that is explained. I am rooted and cleaned out some apps I don't want. I'm particularly careful to not mess things up and noticed the change in behavior on TMO. Wanted to make sure something I hacked for Sprint didn't FUBAR the connection.

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5th March 2014, 01:40 AM |#5  
Jooosty's Avatar
Senior Member
Thanks Meter: 99
I noticed this too on ATT.

When in calls it drops to H .

Thanks for the explanation

Sent from my Nexus 5 using xda app-developers app
5th March 2014, 12:09 PM |#6  
kendong2's Avatar
Recognized Developer
Thanks Meter: 59
It is called CSFB (circuit switched fallback), which will soon be replaced by VoLTE (Voice over LTE), which will be a plain and simple SIP-controlled VOIP call using LTE as carrier. Not sure if current phones can and/or will be upgraded to be able to do VoLTE, or if todays LTE handsets are stuck falling back forever...
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